This invention relates to a grout injection method and apparatus for injecting a grout into the ambient earth.
In general, a grout injection method has been widely employed for the stabilization of a poor subsoil and after many changes have been developed. Among these changes, the view point about the penetration of a grout has also been changed. First, it was considered that the penetration of the grout into the soil should be effected slowly to provide effective stabilization of the ground, and a grout having a gellation time of at least 60 seconds, usually, as long as several minutes to several tens of minutes was employed. In fact, this method is very effective for the stabilization of a homogeneous sandy soil. However, such a slow-curable grout having a long gellation time does not work effectively in a heterogeneous poor subsoil such as a diluvium deposit or an alluvium deposit, a sandy soil abundantly containing ground water, or a complicated ground condition with cohesive soil mingled. Especially, the slow-curable grout may often be diluted by ground water contained in the soil, during the grout injection operation, to such an extent that the desired compression strength of the stabilized mass cannot be developed, or it may escape with the ground water from the area to be treated, or flowed out the surface through a gap between an injection pipe and a wall of a bored hole. Furthermore, the slow-curable grout requires a long hardening time and accordingly requires a long standby time. For these reasons, the slow-curable grout is not always effective and not economical.
To solve these problems involved in the slow-curable grout, a novel method was developed by the inventors of the present invention around 1975. According to this method, a grout having an extremely short gellation time, i.e., gellation time of several seconds (hereinafter referred to as "instantaneously-curable grout" or "flash-curable grout") is employed, and the grout is injected into the soil, using an injection apparatus having a dual-pipe structure. This novel method has been employed since then, prevailing over the conventional method as described above. As disclosed in Japanese Patent Publication No. 38448/1980, this method uses an injection apparatus provided with a spool valve which is fitted in an inner pipe member of the double-pipe boring and injecting pipe. The spool valve is normally in an unoperated position, so as to jet a boring water pumped into a first flow passage formed in an outer pipe member, through a nozzle provided at a tip end of the boring and injecting pipe. During the injection operation, a grout comprised of two liquids which are hardened when combined (hereinafter referred to as "two-liquid type grout") is fed through the first passage and a second passage, respectively, so that the spool valve is pressed down by the liquid fed through the second passage to feed the liquid into a mixing space. The liquid fed through the first passage, is prevented from flowing to the nozzle but is allowed to flow, from the first passage, into the mixing chamber. Thus, in this mixing space, the two liquids are combined, contacted and mixed with each other.
The inventors of the present invention have confirmed, through results of various workings, that this method can provide effective stabilization of heterogeneous ground in a limited area around an injection orifice of the injection pipe. On the other hand, R. H. Karol reported on the phenomena of penetration of a flash-curable grout in Journal of the Soil Mechanics and Foundations Division of ASCE, April 1961 and January 1968. The results of the workings by the inventors are in accordance with the conclusion derived from the study on the penetration phenomena by R. H. Karol et al.
However, the method developed by the inventors of the present invention and disclosed in Japanese Patent Publication No. 38448/1980 still has some disadvantages to be improved. First, the mixing chamber should be provided within the injection pipe so as to allow the liquids to be combined, contacted and mixed in the mixing chamber before injection of the grout. Second, the liquids cannot always be mixed sufficiently because the mixing space of the conventional injection pipe is a narrow, restricted space and is located on one side of the injection pipe and, in addition, the liquids are discharged in linear forms to be combined with each other. Third, the injection pipe should be rotated around its axis, during the injection operation, for providing uniform solidification around the injection pipe, because only one injection orifice is provided in the injection pipe. Fourth, the spool valve does not always operate smoothly, because the liquid in the first passage is forced to flow around or through the spool valve.
The inventors have made intensive and extensive study with a view to solving the problems as described above and found that (1) the structure can be simplified and the operation of the spool valve can be more smooth by an arrangement in which the liquid in the first passage communicates directly with the mixing chamber instead of introducing the liquid into the mixing chamber from a by-pass formed between the outer and inner pipe members, traversing the spool valve, and (2) a plurality of injection orifices can be provided by providing an annular mixing chamber, to enable uniform injection in the limited area around the injection pipe, without rotating the injection pipe around its axis. On the basis of these findings, the inventors have made the present invention.